Metallized films are thermoplastic films having deposited (e.g., by vacuum) thereon a layer of metal such as aluminum. They are used in the food packaging industry as moisture and gas barriers. Metallized films can comprise polyesters such as polyethylene terephthalate (PET) or polypropylene (PP) as the thermoplastic film component. Polyethylene (PE) is also sometimes metallized. Often, metallized films are combined with other materials such as polyethylene (PE), oriented polypropylene (OPP), oriented PET (OPET), paper and paperboard in multilayer structures. These other materials may serve as, for example, abuse layers or sealant layers. They may also provide stiffness and a surface for printing. Thus, multilayer structures comprising metallized film layers adhered to thermoplastic compositions and other substrates can be used as packaging films. Applications include many dry food packages such as powdered drink mix pouches as well as non-packaging applications. Examples of the abuse and/or printed layer include oriented polyethylene terephthalate, polypropylene, either oriented or cast, high density polyethylene (HDPE), paper, paperboard, and biaxally oriented nylon. Examples of the sealant film include low density polyethylene (LDPE), ethylene/vinyl acetate (EVA) copolymers, ionomers (i.e. partially neutralized ethylene/acid copolymers), linear low density PE (LLDPE), and very low density PE (VLDPE).
An adhesive can adhere the metallized surface of the film to the adjoining layer. For example, low-density polyethylene (LDPE) is used as an adhesive layer. Other adhesive layers include ethylene acid copolymers. See e.g., U.S. Pat. No. 6,165,610.
Adhesion of substrates to metallized film using LDPE can be accomplished by processing at high coating temperatures (300 to 330° C.) so that a portion of the polyethylene oxidizes. Oxidization of the LDPE creates polar species that provide moderate adhesion to the metallized surface.
A problem associated with using LDPE to extrusion laminate PE-films to metallized films is that the adhesion of the metallized layer to its base film “ages down” with time. For example, adhesion of LDPE to metallized film is only marginal to begin with, but over a time period of one to several weeks, the bond strength often declines to a level that is no longer functional for the application. One explanation may be that the aging is associated with secondary crystallinity of the LDPE. During the lamination process, the LDPE is quenched very quickly and little primary crystallization can occur. Over time, small “secondary” crystals may form. As PE crystallizes, it shrinks. Shrinkage can put a stress on bonds and reduce peel strength. The bond between the vacuum deposited metallized layer and its base film is the weakest of the bonds in the structure.
More polar polymers adhere to metallized film more readily than less polar materials. Polar polymers that adhere well to metallized film may not adhere well to nonpolar polymers. Therefore, a tie layer providing a balance of properties that allow it to adhere both to metallized film and to non-polar polymers is desirable.
Chemical primers, used to promote adhesion to thermoplastic film substrates, add costs and may cause environmental concerns with solvent-based systems. It is desirable to develop composition or method for adhering metallized film to film or polymers without using primers.